This application claims priority on Japanese Patent Application No. 11-321621 filed on Nov. 11, 1999 and Japanese Patent Application No. 2000-293719 filed on Sep. 27, 2000.
1. Field of the Invention
The present invention relates to a battery pack containing a plurality of cells for use with a charger or battery-powered devices, such as power tools, as a power source. More particularly, the invention relates to a battery pack with an improved structure for cooling the cells during a charge.
2. Description of the Related Art
A conventional battery pack includes on the top surface thereof a mounting portion which in turn includes electrical terminals for the establishment of electrical contact between the battery pack and a charger or a battery-powered device, such as an electric power tool. The battery pack can be recharged by attachment of its mounting portion onto the charger, and also can be used as a power source by attachment of the mounting portion onto a battery-powered device. However, the cells of the battery pack tend to generate heat during each charge, which results in degradation of the cells. To avoid such a disadvantage, a means for cooling the battery cells is typically provided in the battery pack. For example, disclosed in Japan Published Unexamined Patent Application No. 11-219733 is a battery pack which is provided with a cooling structure therein. The structure includes air passages running through the battery pack case and along and between the cells within the battery pack and a plurality of apertures forming intake and discharge ports provided at the top and bottom portions of the battery pack, whereby the intake ports are disposed at a mounting portion to which a charger is attached, with the air passages in communication with both the intake and discharge ports. Thus, cooling air from a blower or fan incorporated in the charger can be introduced from the intake ports to be sent into the interior of the pack through the air passages and outside the pack from the discharge ports so that the heat generated by the cells during a charge is dissipated therefrom.
While the foregoing battery pack with a cooling structure for the battery cells contained therein achieves its intended objective, it is not free from certain problems and inconveniences, thus leaving room for improvement. For example, in the above described structure, the temperature of cooling air drawn into the battery pack case increases due to heat exchanged by the cells as the air flows downstream, i.e. further in the direction of the air flow, thus resulting in a decreased cooling effect of the air downstream. In addition, as the cooling air comes into direct contact with the cells for cooling, the area of contact with the cooling air varies from cell to cell, such that the cells cannot be evenly or equally cooled. Such an uneven cooling effect often results in certain cells having higher temperatures than those of others, thereby allowing the cells with higher temperatures to reach the end of their service life faster than other cells. This by extension shortens the service life of the entire battery.
The cells in a battery pack may be unevenly or unequally cooled due to other causes. For example, a cell or cells may experience a greater heat buildup if surrounded by other cells, thus creating an uneven temperature condition in the cells whether the surrounded cell or cells are located relatively downstream or upstream in the cooling airflow.
In view of the above-identified problems, an important object of the present invention is to provide a battery pack that can suppress increases in temperature of the cooling air downstream in the cooling airflow so as to produce an even cooling effect on all the cells contained in the battery pack.
Another object of the present invention is to provide a battery pack that can ensure a proper temperature balance among the cells in order to prolong the service life of the cells contained therein.
Still another object of the present invention is to provide a battery pack that can more effectively counter variations in temperature of the cells due to, for example, increases in temperature of the cooling air in the pack in order to prolong the service life of the cells and thus the battery pack.
Yet another object of the present invention is to provide a battery pack in which the temperature of all the cells, including those surrounded by others, is maintained at about the same level, preventing shortened service life of certain cells in the battery.
The above objects and other related objects are realized by the invention, which provides a battery pack comprising: a case containing a plurality of cells; at least one air passage formed within the case for allowing cooling air outside the case to enter the case, pass along and/or between the cells, and exit from the case, and at least one radiator provided in the at least one air passage so as to be in contact with outer surfaces of the cells, wherein the heat capacity of the at least one radiator increases in the downstream direction of a flow of the cooling air. By the provision of a radiator in the at least one air passage so as to be in contact with outer surfaces of the cells in such a manner as to increase the heat capacity of each plate in the downstream direction of cooling airflow, the battery pack can effectively control increases in temperature of the cell, and maintain a proper temperature balance among the cells by the cooling effect of the at least one radiator, thereby prolonging the service life of the entire battery.
According to one aspect of the present invention, the aforementioned increase in the heat capacity is achieved by increasing at least one of the surface area and the volume of the at least one radiator. By increasing the surface areas and volumes of the at least one radiator as above, the heat capacity of the radiator can also be easily increased in the downstream direction.
According to another aspect of the present invention, each radiator has a cross-section, transverse to the direction of the flow of the cooling air, that progressively increases in size along the direction of the flow.
According to still another aspect of the present invention, each radiator is contoured to conform to the outer surfaces of the cells. By conforming the surfaces of each radiator to the outer surfaces of the cells, the cells can be more evenly and efficiently cooled.
According to one feature of the present invention, each radiator is a generally rectangular plate having an inner surface contoured to conform to the outer surfaces of the cells and an outer surface opposite the inner surface, with the outer surface of each plate having a plurality of radiator fins thereon protruding into the respective air passage.
According to yet another aspect of the present invention, the radiator fins includes a plurality of horizontal fins with different lengths. The fins are arranged in parallel both with respect to any of the other fins and to the direction of the cooling airflow such that the heat capacity of each radiator increases in the downstream direction of the cooling air.
The invention is also directed to a battery pack comprising: a case containing a plurality of cells; at least one air passage formed within the case for allowing cooling air outside the case to enter the case, pass along and/or between the cells, and exit from the case; and radiator means provided in the at least one air passage so as to be in contact with outer surfaces of the cells, the radiator means having portions each of which corresponds to at least one of the cells, wherein the portions have different heat capacities according to the heat conditions of the corresponding cells.
This arrangement can more effectively counter variations in temperature of the cells that occur due to, for example, increases in temperature of the cooling air in the pack, maintaining a proper temperature balance among the cells. Accordingly, this counters the problem of certain cells reaching the end of their life span faster than others, thus increasing the service life of the entire battery even more effectively. Moreover, the charging time is shortened as the cells of this embodiment are well protected against excessive heat buildup. That is, in conventional battery chargers, the charging circuitry is protected by a reduction in the charging current, which results in longer charging times. However, the relationship between temperature and charging time also means that cooling allows the charging current to be similarly increased, thus shortening the charging time.
According to still another feature of the present invention, the heat capacity of each portion of the radiator means is determined by the area of contact of the portion with the corresponding cell.
Alternatively, the heat capacity of each portion of the radiator means is determined by the thickness of the portion.
Alternatively, the heat capacity of each portion of the radiator means is determined by the material of the portion.
Moreover, the heat capacity of each portion of the radiator means is determined by any combination of the area of contact of the portion with the corresponding cell the thickness of the portion, and the material of the portion.
According to yet another feature of the present invention, the cells are divided into at least one first cell group and at least one second cell group, each cell group including at least one cell and having different heat conditions, and the radiator means includes a plurality of radiator plates having different heat capacities, each radiator plate being in contact with one of the first and second cell groups. Proper heat capacity can be even more easily and accurately determined for each portion of the radiator means.
According to one practice of the present invention, the battery pack comprises one first cell group located generally in the center thereof and two second cell group opposing the first cell group along branches of the at least one air passage.
According to another practice of the present invention, the radiator plates include a first radiator plate and two second radiator plates, the first radiator plate surrounding the first cell group around its entire periphery and each of the second radiator plates abutting inner surfaces of one of the second cell groups, and wherein the first radiator plate has a smaller heat capacity than each of the second radiator plates.
According to still another practice of the present invention, the first radiator plate is made of a synthetic resin and each second radiator plate is made of at least one metal selected from a group consisting of aluminum, copper, and iron.
According to yet another practice of the present invention, the portions of the second radiator plate become thicker in a stepwise manner along the downstream direction of a flow of the cooling air.
In one aspect, the first radiator plate and the second radiator plates have a plurality of portions corresponding to different cells and adapted to remove heat from the corresponding cells, the portions having different heat capacities according to the heat conditions of the corresponding cells.
In another aspect, the heat capacity of each portion of each second radiator plate is determined by the area of contact of the portion with the corresponding cell.
In one embodiment, the heat capacity of each portion of the first and second radiator plates is determined by the thickness of the portion.
In another embodiment, the heat capacity of each portion of each second radiator plate is determined by any combination of the area of contact of the portion with the corresponding cell and the thickness of the portion.
In accordance with another aspect of the present invention, the second cell groups are symmetrical and include an equal number of cells and the first cell group includes fewer cells than each of the second cell groups.
In accordance with still another aspect of the present invention, each second cell group has inner surfaces in contact with one of the second radiator plates, and each second cell group is arranged in a single row of cells bent at one intermediate cell toward the inner surface, the portion of each second radiator plate corresponding to the intermediate cell including two bulges adjacent to the intermediate cell.
In accordance with yet another aspect of the present invention, each bulge is thicker than the remainder of the second radiator plate.
Other general and more specific objects of the invention will in part be obvious and will in part be evident from the drawings and descriptions which follow.